I have only ever built one WTC before, and that was a small one with the electronics thrown in without though.
So I have never built a proper WTC and would like to know if I have the layout of the WTC correct, or if there is some tips and tricks im missing.

Also consider moving the receiver to the other end, so you only have to run the majority of your servo leads a short distance. You could also move the bow servo to the back and run a longer control rod to the front planes. That would keep all the equipment at one end, and the front cap could be bonded in.

When designing the rack, make use of the torsional strength of the Engel tank. it's a heavy and sturdy item, so you might as well make use if it as a structural member.

If you made the pipe 11cm diameter you could use normal PVC waste pipe- very tough, cheap and easy to work.

Bear in mind that when running an external battery you will almost certainly have to extend the battery cables beyond the usual 20cm or so. To avoid damage to your ESC consider adding some extra low ESR caps to the power lines, adjacent to the ESC.

I see. You'll almost certainly need to add some extra buoyancy in the bow to align the forces then. (foam, a smaller cylinder etc.)

You could shorten the aft end of the cylinder, as you have enough air space to compress the piston into. If you need room for the rod to retract, you could bond in a much smaller tube into the end cap for clearance- that's much easier to compensate for than a larger diameter pipe.

Also when designing the rest of the tech rack, think of a structure. e.g. a box section is much sturdier than a few lateral rods, and can be made much lighter too.

A good example of this would be a cardboard box e.g. a packet of cornflakes With all sides buttoned up, it's remarkably stiff and rigid. Open one end to get at the contents, and the box becomes torsionally flimsy, yet still weighs the same.

It's far easier to run servo wires from one end of the dive module to the other than to run a servo linkage. I'd leave your desgin for the servos the way it is.

What kind of ballast system are you planning to use? Depending on type chosen, you might want to consider shortening the dive module tube. The trade-off will be the need for more floatation (e.g. foam) as Andy noted.

Engel recommend that the tank should be no more than about 15% of inside volume of the WTC. This is to keep pressures to sensible limits, which protects the hull seals and also prevents the tank pulling a lot of current, which goes up a lot when it has to compress against higher pressures.

I think you could go to about 20%, but would be cautious about going further than that.